You specifically remember leaving a form field blank on a web page three years ago?
I would have remembered when the message to tag the location of my photos came up.
It didn't work like that. It was a plain blank text field among a couple other blank text fields. No search was brought up upon data entry. Very innocuous; Perhaps that why you don't remember it.
Believe me, I would have remembered doing it. I wouldn't have cared about a location tagging message if I had entered the location manually. Not to mention I couldn't even remember the names of some of the cities where the photos were taken until Facebook suggested them to me.